Electric protective device



May 12, 1 942.

.A. G. ST'IMSON 2,282,833-

ELECTRIC PROTECTIVE DEVICE Filed May 27, 1941 Inventor:

AHen Gflsthhson,

ttornqg.

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Patented May 12, 1942 2,282,833 ELECTRIC PROTECTIVE DEVICE Allen G.Stimson, Marblehead, Mass., assignor to General Electric Company, acorporation of New York Application May 27, 1941, Serial No. 395,431

7 Claims.

My invention relates to electric protective devices and particularly tothermally and magnetically responsive overload relays.

One object of my invention generally stated is to provide an overloadrelay operable upon both alternating current and direct current whichshall be composed of a minimum number of parts and thus be inexpensiveand easy to manufacture, and which shall be efficient in operation.

A further object of the invention is to provide an overload relay whichshall be suitable for the protection of small motors and other electricen- I ergy translating devices against both moderate and extremeoverload conditions.

It is a still further object of my invention to provide a new andimproved overload relay having a thermal response with an inversetime-current characteristic upon the occurrence of a continued moderateoverload below a predetermined value and having magnetic response withan instantaneous characteristic uponthe occurrence of an extremeoverload above said predetermined value.

Another object of the invention is to provide a relay of the abovecharacter which shall b adjustable over a wide range of current values,thereby to facilitate its application to various electrical deviceshaving widely diiferent normal current ratings.

According to the invention, a relay comprising a magnetic core and amagnetic armature normally spaced apart is provided with an energizingwinding composed of a thermo-magnetic material arranged normally toshunt the major portion of the flux away from the armature. Thethermo-magnetic material of which the energizing winding is formed hasthe property of showing a marked decrease in magnetic permeability uponan increase in its temperature due to the flow of excessive currenttherethrough. Upon an increase in the reluctance of the flux leakagepath through the material of the energizing winding, the main portion ofthe flux is forced to pass through the relay armature, thereby toattract the armature to the core and actuate a pair of circuitcontrolling contacts. To facilitate instantaneous response upon theoccurrence of an excessive overload above a predetermined value, aportion of the core is made of reduced cross section, as will be morefully described hereinafter.

My invention itself may be better understood and its object andadvantages further appreciated by referring now to the followingdetailed specification taken in connection with the accompanyingdrawing, the single figure of which is a cross-sectional view of anoverload relay embodying my invention.

In the drawing the relay is represented as comprising a hollow core illof magnetic material,

a magnetic frame ii, an armature I 2 pivotaliy mounted upon the frame IIand biased by a spring l3 to a normal position spaced from the core andan energizing coil l4 wound upon the core. The magnetic frame llprovides a return path for the magnetic flux passing through the corel0, and to this end it comprises a base portion and two side portionsextending in parallel spaced relation to the core Hi. If desired, theside portion of the magnetic frame Il may be made annular therebycompletely to enclose the energizing coil I4. I wish to have itunderstood, however, that, while desirable, it is not essential to thepractice of my invention that the side portions of the magnetic frame llform a closed annulus surrounding the coil.

The magnetic armature I2 is pivotally supported upon a fixed knife-edgedbearing l5 which I have illustrated as forming an integral part of oneside of the frame II. The armature i2 is biased to a normal positionspaced from the core Ill by means of a tension spring l3 connectedbetween one end of the armature l2 and'a fixed point such as a bracket I6 mounted upon the frame H. In order to provide for control of thenormal current rating of the relay, the armatuer i2 is provided with anadjustable stop screw I! mounted in a fixed support l8 which may beconnected to the frame II. In order to control a protective circuit, themovable armature I 2 carries one or more contacts 19 arranged forcooperation with one or more fixed contacts 20.

For purposes of illustration I have shown the contacts I9 and 20 closedin the deenergized position of the relay, but it will be understood ofcourse that, if desired, the fixed contact 20 may be arranged toengagethe movable contact 19 when the armature I2 is in its attractedposition.

The energizing winding ll is designed to carry the load current of anelectric circuit to'be protected, and is preferably formed of a strip ofthermo-magnetic material edgewound upon the core ID. The radialdimension of the winding I4 is preferably such that the winding fillssubstantially the entire space between the core I!) and the magneticframe II, therebyto provide a flux leakage path radially through theturns of the coil from the core to the frame. Preferably also a thinstrip of insulating material (not shown) is provided to separate thewinding II from the core l0 and the frame II.

The winding It may be composed of any alloy having a substantial degreeof magnetic permeability and a high degree of sensitivity ofpermeability to change of temperature within a relatively narrow band oftemperature. Such materials are known as Curie metals, and demonstrateas their essential characteristic a marked change in magneticpermeability within .0 a relatively narrow band of temperature lyingwithin the normal operating range of electrical apparatus. I prefer touse a metal which has a substantial degree of permeability at normaloperating temperatures but becomes substantially non-magnetic whenheated to approximately 100 or 150 centigrade, depending upon theparticular alloy used. It is material of this character to which I referas thermo-magnetic material.

In order to adapt my relay to high current ratings relative to'theoverallsize of the device, I prefer to wind the energizing coil H of abimetal strip comprising a portion Ha of the thermomagnetic material anda portion Nb of low resistance electrically conducting material such ascopper. Preferably the bimetal ,coil is formed by copper plating a coilof thermo-magnetic material. may be easily controlled.

In operation, current passing through the energizing winding l4 sets upa flux in the magnetic core H). In a simple relay this flux wouldordinarily take a return path through the magnetic armature l2 and themagnetic frame ll.

By this methodthe overall resistance In my relay, however, thepermeability of the edgewound coil H of thermo-rnagnetic material issuch that at ordinary temperatures the coil provides a flux leakage pathradially through the turns of the coil from the core I0 to the frame ll.Upon the occurrence of a moderate continued overload, the winding llbecomes heated by the current passing through it and ultimately reachesa temperature at which the thermo-magnetic material becomessubstantially non-magnetic and no longer provides a flux leakage path.In this condition the main portion of the flux passes from the core l0through the armature l2 and the frame II. The fiux now passing from thearmature 2 to the pole face 2| of the core'lB sets up a magneticattraction'which causes the armature to move to its attracted positionagainst the bias ,spring l3.

It will be readily understood by those skilled in' the art that undercontinuous moderate'overload conditions the relay will operate with atime delay which is dependent upon the value of the current in theenergizing winding ll. Relatively large overload currents will requireless time to heat the thermo-magnetic winding II to its demagnetizingtemperature than will smaller overload currents.

Upon the occurrence of a sudden excessive overload of a predeterminedvalue, for example 300 to 500% normal current, the leakage path radiallythrough the winding l4 becomes saturated and sufficient flux is forcedthrough the armature l2 to pick up the armature even before thethermo-magnetic winding H is heated. The relay thus loses its thermalinverse timecurrent characteristic upon the occurrence of suchpredetermined excessive overload and operates substantiallysimultaneously due to the fact that saturation of the flux leakage pathmakes unnecessary the attainment of the demagnetizing temperature. Itwill be understood that a relay having predetermined dimensions willbecome instantaneous 'in its operation, at such a value of overloadcurrent as is sufficient to force a flux of operating value through thearmature 12 without first heating the winding N. The value of thispredetermined overload current is controlled by the'size andsaturability of the winding l4,.the saturability of the core I0, and theair gap between core and armature. The value of the instantaneous tripcurrent is not, however, affected by the resistance of the winding ll,so that the addition of copper plating to the coil will have no efiectin this respect.

It has been found that with a solid magnetic core it is diflicult to setthe current value for instantaneous trip higher than approximately 250to 300% normal rating. In order to adapt the relay for thermal inversetime-current operation up to as high as 500% of its normal currentrating, the cross-sectional area of the core "I may be markedly reducedin a region 22 near the pole face 2| of the core. As shown, the core I0is centrally bored in the region 22. Such reduction of thecross-sectional area of the core makes the core saturable in the regionof the pole face 2 I, and this in turn reduces the relative effect ofapproaching saturation of the thermo magnetic material of the energizingwinding. For example, if, upon the occurrence of a 300% overload with asolid core relay, the leakage path through the edgewound coil l4 wouldsaturate to such a point that suflicient flux would pass through thearmature l2 to attract the armature, concurrent saturation of a hollowcore upon the same overload would simultaneously tend to restrict theamount of flux passing through the armature l2 and force more of thefiux through the partially saturated leakage path. Thus the currentvalue for instantaneous trip is raised by making the core tip saturable.

The copper portion Nb of the energizing winding l4 permits higher normalcurrent ratings by decreasing the overall resistance of a coil of apredetermined size. It will also be found that the tripping time at highoverloads will be slightly increased by the use of a bimetallic coil dueto the time delay introduced by the conduction of heat from the copperportion of the coil to the thermo-magnetic portion. The copper portionof the coil acts substantially as a shunt heater for the thermo-magneticportion.

The device disclosed in the drawing is an automatically resetting typeofrelay since the armature l2 will assume its deenegized position upondecrease of current and temperature in the winding [4 to their normalvalues. The value of the resetting current may be adjustablypredetermined by providing an adjustable stop on either the core ID orthe armature l2 arranged to control the air gap between the armature andthe pole face 2| when the armature is in its attracted position.

If it is desired to enforce manual resetting of the relay after theoccurrence of an overload, latch means may be provided for engaging thearmature l2 in its attracted position and holding it until manuallyreleased.

While I have described only one preferred embodiment of my invention byway of illustration, many modifications will occur to those skilled inthe art and I therefore Wish to have it'understood that I intend in theappended claims to cover all such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A thermally responsive overload relay comprising a magnetizable core,a magnetizable armature positioned in attractive relationship to saidcore, means providing a substantially closed flux path including saidcore and said armature, and an energizing coil of thermo-magneticmaterial arranged normally to provide a itiux leakage path in shuntrelation to said armaure.

2, An overload relay comprising a magnetizable core, a magnetizablearmature biased to a normal position in spaced attractive relationshipto said core, a magnetizable frame arranged in spaced relation to saidcore to provide a return path for magnetic flux passing through saidcore and armature, and means for normally shunting the main portion ofsaid magnetic flux away from said armature comprising a current carryingwinding of thermo-magnetic material encircling said core and extendingradially between said core and said frame.

3. An overload relay comprising cooperating circuit controllingcontacts, a magnetizable core, a movable armature of magnetizablematerial arranged to control one of said contacts, means for biasingsaid armature to a normal position in spaced attractive relationship tosaid core, said core being formed to provide saturable section adjacentsaid armature, a magnetizable frame having an end portion and a sideportion providing a return path for magnetic flux passing through saidcore and armature, and means for normally shunting the main portion ofsaid magnetic flux away from said armature through a leakage pathbetween said core and said frame comprising an energizing winding forsaid relay composed of a thermo-magnetic alloy arranged to encircle saidcore and to fill substantially the entire space between said core andsaid frame, whereby continuous overloads below a predetermined valuewill heat said thermo-magnetic material to a predetermined demagnetizingtemperature thereby to disable said leakage path and actuate said relaywith an inverse time-current characteristic and a sudden overload abovesaid predetermined value will actuate said relay instantaneously. p

4. An overload relay comprising cooperating circuit controllingcontacts, a magnetizable core having an end portion defining a poleface, said core being of substantially diminished cross-sectional areain the region of said pole face, a movable magnetizable armatureoperatively associated with one of said contacts, spring means rorbiasing said armature to a normal position from said pole face, amagnetizable irame having a side portion magnetically linked with oneendof said armature and spaced from said core and an end portionmagnetically linked with said core thereby to provide a return path formagnetic flux passing through said core and armature, and means fornormally shunting the main portion of said magnetic flux away from saidarmature comprising an energizing winding formed of thermo-magneticalloy, said energizing winding being arranged to encircle said core andto substantially fill the space between said core and said frame.

5. An overload relay comprising cooperating circuit controllingcontacts, a magnetizable core member, a magnetizable armatureoperatively associated with one of said contacts and biased to normalposition in spaced attractive relationship to said core member, amagnetizable frame magnetically linked with said armature and core toprovide a return path for magnetic flux passing through said core andarmature, said frame having a side portion positioned in substantiallyparallel spaced relation to said core, and a bimetallic energizingwinding for said relay comprising a strip of thermo-magnetic materialencircling said core and extending radially from said core to said sideportion of said frame thereby to provide a low reluctance leakage pathbetween said core and said frame at normal operating temperatures, saidwinding also including a strip of relatively low resistance electricallyconducting material formed integrally with said thermo-magnetic materialthereby to decrease the overall resistance of said winding.

6. An overload relay comprising cooperating circuit controllingcontacts, a magnetizable core member, a movable magnetizable armatureoperatively associated with one of said contacts and biased to a normalposition in spaced attractive relationship to said core member, amagnetizable frame providing a return path for magnetic flux passingthrough said core and armature, said frame member including an endportion magnetically linked to said core and an annular side portionmagnetically linked to said armature and arranged in parallel spacedrelation to said core, and a bimetallic energizing winding for saidrelay encircling said core and extending radially to fill substantiallythe entire annular space between said core and said annular side portionof said frame, said bimetallic winding comprising a strip ofthermo-magnetic material providing a low reluctance leakage path betweensaid core and said frame at normal operating temperature and anintegrally formed strip of low resistance electrically conductingmaterial for decreasing the overall resistance of said winding.

7. An overload relay comprising cooperating circuit controllingcontacts, a central magnetizable core member having an end portiondefining a pole face, said core member being centrally bored in theregion of said pole face to provide a saturable section of a relativelysmall crosssectional area, a magnetizable frame having an annular sideportion in concentric relation to said core and an end portionmagnetically linked to said core thereby to provide a return path formagnetic flux passing through said core, a mag netizable armaturepivotally mounted upon said side portion of said frame and operativelyassociated with one of said contacts, spring means connected betweensaid armature and said frame to bias said armature to a normaldeenergized position in spaced attractive relationship to said poleface, an adjustable stop for detern'iining the deenergized position ofsaid armature with respect to said pole face, and a bimetallicenergizlng winding for said relay encircling said core and extendingradially to fill substantially the entire annular space between saidcore and said annular side portion of said frame, said windingcomprising an edgewound strip of thermo-magnetic material providing alow reluctance leakage path between said core and said frame at normaloperating temperatures and an integrally formed portion of lowresistance electrically conducting material for decreasing the overallresistance of said winding, said leakage path being saturable upon theoccurrence of a sudden overload in excess of a predetermined valuethereby to provide instantaneous actuation of said armature.

ALLEN G. STIMSON.

